Flow cytometry is used in the diagnosis and follow-up of lymphoproliferative diseases, including Large Granular Lymphocytic Leukemia. A unique treatment trial utilizing cyclosporine has been undertaken by the Hematology Branch of NHLBI and has been successful in decreasing some of the cytopenias associated with this disease. Since June, 1997, the Hematology Service of CPD has begun performing follow-up flow cytometry of these patients during the course of their treatment and will be correlated with their responses.